Abstract

Results of a detailed geological and mineralogical investigation of siderite microconcretions are presented. These are confined to an intercalation in coal seam No. 559 of the Karviná Formation (Bashkirian Stage of Pennsylvanian) in the Czech part of the Upper Silesian Basin. The observed coal seam was encountered in the Lazy Mine situated in the Karviná section of the basin (Fig. 1). Coal seam No. 559 belongs to the uppermost part of the Saddle Member of the Karviná Fm. that is typical of its coarse-grained lithology. Sandstones and conglomerates form 70 to 85 vol.% of the total unit thickness, whereas siltstones and coal seams make up the remaining part of local lithology. Thin intercalations in coal seam were found usually to consist of clay minerals, and a few chunks of volcaniclastic sedimentary rocks were also identified. An intercalation comprising carbonate microconcretions was found in one coal face and also in five underground exploratory boreholes (Fig. 2). It occurs approximately in the middle of the coal seam profile (Figs 3 and 4), and its thickness varies between 3 cm and 8 cm. The X-ray powder diffraction method (PXRD) showed the bulk of the intercalation to contain siderite, muscovite, quartz, kaolinite, to a lesser extent also K-feldspar, a mineral of plagioclase series, and anatase. Concretions form mostly spherical aggregates ranging from 0.X to 3 mm in size (Fig. 5a). The wavelength-dispersive X-ray spectroscopy (WDS, Table 1) proved that nodules consist of siderite (90-92 wt.%), with an admixture of rhodochrosite (up to 4 wt.%), magnesite (max. 3 wt.%), and calcite (up to wt. 6%). Although the nodules exhibit distinct zoning, it is not visible in BSE (Fig. 5), and siderite was found to be chemically heterogeneous. Septarian cracks are filled with almost pure calcite (98 wt.%; Fig. 5b). The origin of these concretions is thought to have taken place during a break in sedimentation of siderite precursor in coal-swamp, long enough to start a nucleation of siderite, probably supported by microbial activity. Spherical shape of concretions does not necessarily indicate their growth prior to the sediment compaction, but rather is a product of recrystallization during diagenetic lithification of the sediment. Since this stage of sedimentary sequence was not observed in other mining works, it is difficult to estimate the original areal extent of this horizon.